Optical energy transfer systems that combine a series of lenses and mirrors have been known for many years, and in fact such systems have played a major role in the commercial development of certain analytical instrumentation. An example of such an instrument is a spectrometer which passes a light beam through a sample cell to measure the absorption spectrum of an unknown gas in a predetermined wavelength region.
Two important characteristics of the types of optical system described herein are that they must be capable of changing the f/# of the beam as it traverses the system while at the same time substantially matching the etendue or optical throughput from one part of the instrument system to another to avoid energy loss through vignetting. This is especially significant in today's commercial optical instruments where limitations of size and cost result in widely varying optical requirements between spatially distinct locations within the instrument.
In the particular example of an infrared spectrometer, it is desirable to obtain the highest signal level from the available source power by passing as much infrared energy as possible into the system through the monochromator slit. Therefore, an input beam with as large a solid angle as possible that does not sacrifice spectral resolution (e.g., f/1.5 for a circular variable filter based spectrometer) is used to form the first image (beam focus) of the source at the slit. The divergence of the beam as it traverses the cell is however more severely limited due to optical aberrations and practical size requirements of the absorption cell itself and the associated optics. Typically, the beam passing into and out of the cell is f/4.5.
The product of the area of the slit and the solid angle of the beam at the slit establishes the optical throughput or etendue of the spectrometer system. For best instrument performance, the etendue in other sections of the system, such as the absorption cell, should be the same such that energy throughput is maximized even though the f/# requirements may vary widely. To minimize vignetting energy losses, pupil dimensions defined in respective sections should be preserved while the substantially different solid angles of the beams in various sections of the device are simultaneously matched.
Optical energy transfer systems of the prior art present certain drawbacks. Particularly when very wide angle or "fast" beams are involved, a conventional lens/mirror system requires strong (i.e., short focal length) lenses to produce a desired f/# change with minimal vignetting. However, such lenses produce aberrations and Fresnel reflections and thus are themselves sources of lost energy for the system. Further these lenses are space-consuming and field lenses as well as focusing lenses are required to achieve the desired result, all of which adds to the overall size and weight of the instrument system.